I've taken prenatal vitamins consistently for the past 13 years, when pregnant with my 4, through breastfeeding and in between. I'm sure there are nutritional gaps in my diet. I buy prenatals OTC. They typically prescribe the vitamins for pregnant AND nursing mothers.
Most people don't have very healthy diets, and taking vitamins ensures mother and baby get all they need. I have to admit I only remember to take them once or twice a week though. They certainly are not a requirement, but they are good for you.